XR77103 Datasheet, PDF (15/28 Pages) Exar Corporation – 3-Output Programmable Buck Regulator

XR77103
Applications Information
Operation
XR77103 is a power management IC with three step-down
buck converters. Both high-side and low-side MOSFETs
are integrated to provide fully synchronous conversion
with higher efficiency. XR77103 can support 4.5V to
14V input supply, high load current, 300kHz to 2.2MHz
clocking. The buck converters have an optional PSM
mode which can improve power dissipation at light loads.
Alternatively, the device implements a constant
frequency mode. The wide switching frequency of 300kHz
to 2.2MHz allows for efficiency and size optimization.
The switching frequency is adjustable by writing data
through I²C. The SYNC pin also provides means to
synchronize the power converter to an external clock signal.
Input ripple is reduced by 180 degree out-of-phase operation
among converters. All three buck converters have peak
current mode control which simplifies external frequency
compensation. Each buck converter has an individual peak
inductor current limit which is set through I²C. The adjustable
current limit enables high efficiency design with smaller and
less expensive inductors. The device has a power good
comparator monitoring the output voltages. Each converter
has its own soft-start independently controlled through I²C.
Continuous Conduction Mode (CCM)
This is a natural mode of a synchronous buck converter.
Advantage of the CCM mode is that the switching frequency
is always constant and allows for better EMI control in
the system. The downside of CCM mode is that at light
loads system efficiency will become lower.
Pulse Skipping Mode (PSM)
In order to improve efficiency at light load the
device implements two functions. Both functions are
enabled simultaneously. One function is a Zero Current
Detect comparator (ZCD) which detects zero current
in the inductor and turns off synchronous MOSFET,
preventing negative inductor current. This ensures that the
device enters DCM mode as the load decreases. In this
mode the device still operates at a constant frequency.
The second function is an internal skip comparator.
This comparator detects low level of output current. If this
low level is detected the device will start to skip pulses.
This is done to improve light load efficiency by effectively
reducing switching frequency. For details contact
[e-mail].
Output Voltage Setting
Output voltage of each converter can be programmed by
I²C interface. It can be set from 0.8V to 6V with
6-bit resolution. The registers 00h to 02h are allocated to
setting each output of the converters. Alternatively, output
voltages can be set externally using external resistor
dividers. Setting EXTx (bit 7) of the registers 00h to 02h
allows external resistor divider for feedback. Output voltage
is determined by the following equation.
VOX = 0.8V x
1+
R1
R2
V_tORXP
=
1
f_SW
x
10⁶
VOX
R1
XR77103
XR77103
R2
Figure 35. Output Voltage Setting
This feature can make the device applicable to AVS
(automatic voltage scaling) system. Output voltage
can be adjusted automatically by external DC voltage.
Figure 36 shows application circuit of supply for
AVS system.
XR77103
VOUTX
VOX
R1
RDAC
R2
AVS SUPPLY
SOC
VDAC _{PVT MNT}
Figure 36. AVS Control
REV1B
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